The present invention relates to a servo-system for use in an automatic balancing instrument or the like.
In prior servo-systems, a direct-current (dc) signal representing the deviation between an input signal and a comparison value is amplified by an alternating current (ac) amplifier after conversion to an alternating current by means of a chopper, and an ac servo-motor is driven to displace the brush of a slide resistor in a comparison-value generator circuit so as to zero-balance the dc deviation signal. The chopper and the ac amplifier are ac coupled to each other through a coupling capacitor. The existence of such a coupling capacitor in a servo-loop causes a phase shift which leads to unstable operation of the servo-loop.
In one recent servo-system disclosed in U.S. Pat. No. 3,866,103, for example, a linear integrated circuit (IC) including a direct-coupled amplifier is employed. However, since the linear IC is used as a high-gain ac amplifier, it becomes necessary to provide a bias stabilizing circuit for applying negative feedback to the linear IC in a dc and low-frequency range, hence complicating the structure. Moreover, the bias stabilizing circuit causes delay in the signal phase which reduces the servo-system gain (motor torque and speed). This phenomenon is particularly regarded as a great disadvantage because it is more conspicuous as the linear IC gain becomes higher. Furthermore, an input variation range is wide in the servo-system of this type. For one automatic balancing instrument, for example, the input span varies in the order of 10.sup.3 times since the minimum span is 5 millivolts while the maximum span is 25 volts. For this reason, the voltage from a slide resistor is introduced as a comparison value through a voltage divider circuit for range adjustment, and the displacement of the slide resistor brush from 0 to 100 percent is made to correspond to the input span. But if the voltage division ratio of the divider circuit is changed for range adjustment, the loop gain of the servo-loop is also changed, and such a change is wide particularly in the automatic balancing instrument. It is desired that the loop gain in the servo-loop be constant despite nonlinearity of the loop and its vibratory transfer function. Accordingly, in the range adjustment performed heretofore, manual operation was necessary to adjust the servo-amplifier gain to avoid a change in the loop gain, and thus there exist some disadvantages including complicated adjustment and increase of the number of component parts to be changed for the range adjustment.
Such problems are solved in the servo-system of U.S. patent application Ser. No. 738,736 now U.S. Pat. No. 4,161,678 filed by the same inventors on Nov. 4, 1976. In that application an input signal and a comparison value are amplified by a dc amplifier employing a linear IC and are then converted to a rectangular-wave signal by a servo-amplifier consisting of a linear IC and a switch, so as to drive an ac servo-motor. As before, in this servo-system two or three stages of low-pass filters consisting of capacitors and resistors are cascade-connected to the input circuit through which the input signal is fed, so as to attenuate commercial-frequency noise superposed on the input signal. The time constant of each filter is selected to be great for the purpose of attaining a high noise attenuation ratio. The existence of any filter of a great time constant in the input circuit is disadvantageous since the input impedance is reduced in a high-frequency range and since the input circuit is restricted by the withstand voltage and leakage current of the filter capacitor. In the instance where the servo-system is formed into a multiplex type to switch the input signal sequentially by a selector switch, there occurs a state where the input is open during the switching action. In such a state the input resistance on the input-circuit side as viewed from the amplifier is widely changed resulting in charging of the filter capacitor to an abnormal voltage. This phenomenon is particularly conspicuous when a direct-coupled dc amplifier is employed because of the amplifier's bias current. And the charge voltage causes malfunctioning of the system since the charge voltage determined by the time constant of the filter is held even after switchover to the next input. In addition to the above, on many occasions where instruments such as controllers and alarms are connected in parallel with the servo-system, a charging current flowing transiently in the filter capacitor at the variation or switchover of the input signal is liable to bring about a disturbance in the parallel-connected instruments. In order to prevent such result, each instrument is generally equipped with an individual filter which further complicates the entire system.
In the servo-system of this type, a thermocouple or a resistance bulb is also used as an input signal source for the servo-system. Therefore, it would be remarkably convenient if such an input signal source could be formed with a minimum change of component parts.
Where a thermocouple is employed as an input signal source, it is desired that the thermocouple output be automatically compensated by an electric signal proportional to the temperature detected at the cold junction, since the thermocouple generates a thermoelectromotive force proportional to the temperature difference between the detecting end and the cold junction.
Where a resistance bulb is used as an input source, a resistance variation in the resistance bulb should be as linear as possible relative to the temperature at the detecting end despite its tendancy to become non-linear. Further, since an error occurs due to the influence of leadwire resistance in the resistance bulb, effective removal of such influence is desirable.